Computer hackers swiped personal information from at least 500 million Yahoo accounts in what is believed to be the biggest digital break-in at an email provider.

The massive security breakdown disclosed Thursday poses new headaches for Yahoo CEO Marissa Mayer as she scrambles to close a $4.8 billion (U.S.) sale to Verizon Communication.

The breach Thursday dates back to late 2014, raising questions about the checks and balances within Yahoo, a fallen Internet star that has been laying off staff to counter a steep drop in revenue during the past eight years.

At the time of the break-in, Yahoo’s security team was led by Alex Stamos, a respected industry executive who left last year to take a similar job at Facebook.